Inductors and transformers are used for signal processing such as processing of gate signals for power devices. In many applications, in particular high power applications, the primary and secondary coil of the transformer can be operated at different voltages. Such transformers need a reliable electrical insulation between the primary and the secondary coil to prevent electrical breakthrough and malfunction of the power devices. For example, a transformer can be used to couple a low voltage control unit for controlling a high voltage device directly with the high voltage device. Other options for coupling a low voltage device with a high voltage device are fibre optics or integrated circuits on SOI-technology.
Transformers are often integrated into the integrated circuit (IC) of the control unit by using the upper metallisation layer of the IC for forming the primary coil. The upper metallisation is covered by an imide layer on which the secondary coil is formed. Such transformers need additional layers and have a limited dielectric strength due to the imide insulation. Furthermore, a thick imide layer may warp the semiconductor substrate of the IC. Moreover, imide or other synthetic materials used as insulation may degas in sputter tools used for forming the secondary coil which causes additional cleaning of the sputter tools.